1. Field of the Invention
The present invention relates to an optical head unit and optical information writing/reading device, and more particularly, to an optical head unit and an optical information writing/reading device having a plurality of objective lenses.
2. Description of the Related Art
A recording density of an optical information writing/reading device is inversely proportional to a square of the diameter of a focal spot formed on an optical recording medium by an optical head unit. In other words, the smaller the diameter of the focal spot, the higher the recording density. The diameter of the focal spot is proportional to a wavelength of a light source and is inversely proportional to a numerical aperture of an objective lens. In other words, the shorter the wavelength of the light source and the higher the numerical aperture of the objective lens, the smaller the diameter of the focal spot. On the other hand, when an optical recording medium (optical disc) is tilted to an objective lens, the shape of a focal spot is fluctuated due to coma aberration, resulting in deterioration of the writing/reading characteristic. The coma aberration is inversely proportional to the wavelength of a light source and is proportional to the cube of the numerical aperture of the objective lens and the thickness of a protective layer of an optical recording medium. When the thicknesses of protective layers of optical recording media are the same, the shorter the wavelength of the light source and the higher the numerical aperture of the objective lens, the margin of the tilt of the optical recording medium with respect to the writing/reading characteristic is narrower.
In an optical information writing/reading device which shortens the wavelength of a light source and increases the numerical aperture of an objective lens to increase the recording density, the thickness of a protective layer of an optical recording medium is smaller to secure the margin of the tilt of the optical recording medium with respect to the write/read characteristic. For example, in a standard CD (compact disc) having a capacity of 650 MB, the wavelength of a light source is 780 nm, the numerical aperture of an objective lens is 0.45, and the thickness of a protective layer of an optical disc is 1.2 mm. In a standard DVD (digital versatile disc) having a capacity of 4.7 GB, the wavelength of a light source is 650 nm, the numerical aperture of an objective lens is 0.6, and the thickness of a protective layer of an optical disc is 0.6 mm.
In a typical optical head, an objective lens is designed to cancel a spherical aberration to an optical disc having a protective layer having a certain thickness. When writing and reading are performed onto/from optical discs having protective layers having different thicknesses, the spherical aberration remains and correct writing and reading cannot be performed. Accordingly, there is proposed an optical head unit having a compatible function which can perform writing and reading onto/from both of an optical disc of the DVD standard and an optical disc of the CD standard. Such kind of optical head units are classified into one using a single objective lens and one using a plurality of objective lenses. The optical head unit using a plurality of objective lenses has a construction slightly more complicated than that of the optical head unit using a single objective lens. However, the respective objective lenses can be optimally designed for the optical discs of the respective standards, so that the writing/reading performance to the optical discs of the respective standards is excellent.
As an example of a prior art optical head unit using a plurality of objective lenses, which can perform writing and reading onto/from both of an optical disc of the DVD standard and an optical disc of the CD standard, Japanese Published Application 2000-242943 discloses an optical head unit shown in FIG. 1.
As shown in FIG. 1, an optical head unit 101 in the prior art has a first optical system 102a, a second optical system 102b, a first objective lens 103a mounted on a first actuator 104a, and a second objective lens 103b mounted on a second actuator 104b. 
The first optical system 102a and the second optical system 102b have a semiconductor laser, not shown, as a light source. A photodetector, not shown, receives a light from an optical disc 105 of the DVD standard or the CD standard. The wavelength of an emitted light of the semiconductor laser in the first optical system 102a is 650 nm, and the wavelength of an emitted light of the semiconductor laser in the second optical system 102b is 780 nm.
The emitted light from the semiconductor laser in the first optical system 102a is incident upon the first objective lens 103a and is focused onto the optical disc 105 of the DVD standard having a protective layer having a thickness of 0.6 mm. A reflected light from the optical disc 105 passes through the first objective lens 103a in the reverse direction to be received by the photodetector in the first optical system 102a. In addition, the emitted light from the semiconductor laser in the second optical system 102b is incident upon the second objective lens 103b and is focused onto the optical disc 105 of the CD standard having a protective layer having a thickness of 1.2 mm. A reflected light from the optical disc 105 passes through the second objective lens 103b in the reverse direction to be received by the photodetector in the second optical system 102b. 
The first objective lens 103a has a spherical aberration canceling a spherical aberration caused when the light with a wavelength of 650 nm passes through the protective layer having a thickness of 0.6 mm of the optical disc 105. The second objective lens 103b has a spherical aberration canceling a spherical aberration caused when the light with a wavelength of 780 nm passes through the protective layer having a thickness of 1.2 mm of the optical disc 105.
The optical head unit 101 is moved between the innermost circumference and the outermost circumference of the optical disc 105 in the direction of the arrows of FIG. 1 along two rails 106a and 106b arranged in parallel with each other.
In FIG. 1, the numeral 131a denotes a first optical path connecting the first optical system 102a and the first objective lens 103a, and the numeral 131b denotes a second optical path connecting the second optical system 102b and the second objective lens 103b. 
In recent years, to increase the recording density, there has been proposed a next-generation standard in which the wavelength of a light source is shorter, the numerical aperture of an objective lens is higher, and the thickness of a protective layer of an optical recording medium (optical disc) is smaller. For example, with the name of Blu-ray Disc (BD), there has been proposed a next-generation standard having a capacity of 23.3 GB in which the wavelength of a light source is 405 nm, the numerical aperture of an objective lens is 0.85, and the thickness of a protective layer (cover layer) of an optical disc is 0.1 mm. Accordingly, what is desired is an optical head unit having a compatible function which can perform writing and reading onto/from any of an optical disc of such next-generation standard and conventional optical discs of the DVD standard and the CD standard. However, there has not yet been an optical head unit using a plurality of objective lenses which can perform writing and reading onto/from any of the optical disc of the next-generation standard and the conventional optical discs of the DVD standard and the CD standard.